Such color printing units are the general state of the art for electrophotographic printers. LED or laser full-color printers are also generally known for large-format printing with a print width >297 mm. These have at least four electrophotographic printing units—one for each color—in the respective printing width. For full-color prints, the colors cyan, magenta, yellow and black are superimposed. The literature, for example, the book “Handbuch der Printmedien” (Handbook of Print Media) by Helmut Kipphan (ed.), Springer-Verlag Berlin Heidelberg 2000, Chapter 5.2.3: “Farbwerk (Entwicklungseinheit) and Toner” (Inking Unit (Development Unit) and Toner), pages 726 to 730, describes a wide range of options for implementation in detail. There is also a large number of large-format color printers on the market using these and similar technologies.
For a more detailed description of the invention, the design type “Tandem architecture with intermediate carrier belt” is now described, as it is described as the state of the art, for example in U.S. Pat. No. 4,903,067 on the basis of its FIG. 9 in particular. With this type of executing the generation of color prints, for each of the four colors, there is one printing unit from the electrophotographic units described above for a monochrome printer, with the exception of the fixation. Thus, a toner image in the respective colors cyan, magenta, yellow and black is developed on each photo conductor. However, the toner is not transferred directly to the substrate, but is transferred only to an intermediate carrier. Such intermediate carrier is usually designed as a special continuous belt. The belt is referred to as the ITB (intermediate transfer belt) or transfer belt. The four colors are then collected and transferred altogether to the substrate in a single step. Both the transfer from the photo conductors to the transfer belt and the transfer from the transfer belt to the substrate is carried out electrostatically, in a manner analogous to the transfer described above for a monochrome process, from the photo conductor to the substrate. Finally, the toner is fixed to the substrate.
The problem with such color prints, especially with large-format printers, is that the transfer belt can run out of the center even after a relatively short period of operation. Such drift of the transfer belt from its straight run can be caused by changing the transfer belt, the misalignment of the printer, fluctuations in temperature and humidity that bring about an expansion of the transfer belt, the swiveling in and out of the various anode rollers, tolerances in the deflection rollers along with pressure on the cleaning unit.
In order to achieve a constant position of the information on the substrate and to avoid image distortion, it must be ensured that the transfer belt ideally runs straight. If the transfer belt “runs away” or drifts to one side, this can lead to the destruction of the transfer belt. Therefore, it is necessary to use a control for the straight run of the transfer belt, especially in view of the fact that from the first to the last printing station, only a maximum pixel offset of 0.1 mm is permissible.
DE 10 2012 104 584 A1 describes a method for controlling a color printer or a color copier with which a color separation of a first color and a color separation of a second color are printed on the substrate to generate a printed image. Furthermore, at least one control panel with a predetermined pattern of the first and second colors is printed. The total color value of the control field is measured with the assistance of a color value sensor. Furthermore, a deviation between the measured color value and a target color value of the control field is determined and, depending on the deviation, a control signal is generated to correct the register error between the two color separations. With such color prints, the substrate is tensioned by means of transport elements.
U.S. Pat. No. 9,335,671 B2 describes a transfer belt unit having a steering mechanism with an automatic alignment method for the transfer belt, comprising a boundary section configured such that a steering roller for correcting the deviation of the position of the transfer belt is rotatable in width around a steering axis line, while the inclination of the steering roller is limited due to rotation.
An image processing device having color difference detection pattern output device is known from DE 69 619 766 T2; this outputs an image signal for generating a color difference detection pattern for detecting periodic rotational variations occurring in the image processing device. The image processing device further comprises a pattern recognition device for detecting the color difference detection pattern on a continuous carrier. Phase detection means recognize the rotational phase of at least one of the image carriers of the image processing device from a detection signal of the pattern recognition device. Finally, the image processing device is provided with rotational phase controller for individually controlling the rotational phase of at least one of the image carriers of the image processing device and the continuous carrier on the basis of the phase information detected by the phase detecting device.
JP 3399492 B2 describes a belt drive control device that features excellent responsiveness and superb accuracy in position correction and suppresses high frequency vibrations in the normal state after rocking motions. In doing so, a position and speed calculation unit determines at least two of the rocking motion variables. The rocking motion changes the variable and the rocking motion speed in response to the detection signal from the position of the transfer belt. A control unit changes the scanning period of the transfer belt or the control amplification based on a result of the calculation by the position and speed calculation unit. A driver circuit generates a control voltage corresponding to the control output signal of the control unit, in order to drive a steering motor.
A device for tracking the position of a moving photoconductive belt and adjusting an imager in an electrophotographic printing machine, in order to correct alignment errors during the formation of a composite image, is known from U.S. Pat. No. 5,394,223. Registration errors are captured by developing a suitable set of target markers, capturing such target markers, and controlling the imager's position. The photoconductive belt is driven, guided and tensioned by a stripping roller, a tension roller, an idle roller and a drive roller.
Thus, that at least one optical sensor is to be arranged on the transfer belt, with the assistance of which, for example, the density and/or position of reference marks printed on the transfer belt can be measured, is known from such citations. With such measured values, various parameters can be determined; these include stitching, register, i.e. the positional accuracy of the partial color prints (color separations) on the printed product in relation to each other (also called “color register”), and the alignment of the stations. The sensor can be fixed in the area of a specially designed, stationary deflection roller, in the area of which the transfer belt is conveyed without any ripples. For accurate measured values, the sensor is positioned opposite a special tension roller in accordance with the state of the art. The tension roller smoothes the transfer belt at this point, such that errors in recording the optical density (for example, due to ripples of the transfer belt) are avoided. However, such a special deflection roller or such a special tension roller represents an additional expense that has a significant impact both in terms of costs and space requirements.
EP 2 028 557 A2 discloses a sensor device that is arranged in the area of the photo conductor drums and that serves to adjust the speed of the transfer belt. An adjusting of the straight travel of the transfer belt with the aid of the measurement results of the sensor device is not activated.
US 2015/205231 discloses a control roller that serves to control or adjust the straight travel of a transfer belt. However, the sensor means used here are edge sensors for determining the positions of the edge of the transfer belt. Such sensors are arranged in an area where the transfer belt is freely tensioned without guidance.